MERLIN LEGEND Communications System Release 6.1
Feature Reference
555-661-110
Issue 1
August 1998
Features
Page 504Primary Rate Interface (PRI) and T1
The following two methods of framing can be used by a 100D module, but the
framing method chosen must match the framing at the far end:
â– D4 Framing Format. The system is factory-set for D4 framing. A D4 frame
consists of 24 eight-bit time slots and one framing bit. To perform
synchronization, the receiving equipment uses the framing information to
identify the start of each frame and to identify which frames contain
signaling information. The framing information repeats once every 12
frames; these 12 frames form the D4 superframe.
â– ESF Framing Format. The extended superframe (ESF) format extends
the 12-frame D4 superframe to a 24-frame superframe. The 24 framing bits
include a cyclic redundancy check (CRC) for the entire ESF and a facility
data link for maintenance. The ESF can detect more errors than D4
framing can. In Release 6.0 and later systems (Hybrid/PBX mode only),
use this format for tandem trunks.
Line Coding 31
The DS1 signal consists of a continuous stream of ones and zeros, encoded into
bipolar pulses for transmission. Only the ones create a pulse; the zeros represent
the absence of a pulse. Pulses alternate between positive and negative. This type
of line coding is called
bipolar
or
alternate mark inversion
(AMI). The line-coding
formats guarantee that the ones-density requirement is met to achieve clock
recovery.
To meet the ones-density requirement, either zero code suppression (ZCS) or
bipolar 8 zero substitution (B8ZS) line coding can be chosen, but the selected line
coding must match the line coding at the far end. In Release 6.0 and later systems
(Hybrid/PBX mode only), use this coding for tandem trunks.
ZCS line coding monitors each DS0 channel and prevents strings of eight or more
zeros. Upon detecting eight consecutive zeros in a channel octet, ZCS line coding
forcibly changes the seventh zero (the second least significant bit) to a one. The
factory-set line coding is ZCS.
B8ZS line coding matches the ones-density requirement by using a special
sequence with a
bipolar violation
in bit positions 4 and 7. Normally, for bipolar
transmission, ones are encoded alternately as a positive then negative, or
negative then positive, pulse. If two positive or two negative pulses are received in
succession, a bipolar violation occurs. Bipolar violations are normally caused by
noise hits to the signal; however, B8ZS uses a specific binary sequence with
bipolar violations as a code for an all-zero channel octet.
B8ZS line coding is preferred over ZCS because it provides no possibility of
corrupting data transmissions.
B8ZS violations are passed by the ACCULINKâ„¢ 3150 and 3160/3164, and
ESF T1 channel service units (CSUs) but not by other CSUs.
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